Horner Syndrome in Dogs and CatsHorner Syndrome is not a disease itself, but a combination of clinical symptoms that can have multiple potential causes. This can affect dogs and cats of any age, from juveniles to seniors, but it is more commonly seen in middle age pets. Golden Retrievers, Dobermans, Labs, Shelties, and Weimaraners seem to be at higher risk. Continue reading to learn more about Horner Syndrome, clinical symptoms, potential causes, tests, and treatment options!FirstVet is the #1 online video veterinary service.FirstVet offers video calls with experienced veterinarians for just $35. You can get a consultation within minutes by downloading the FirstVet app for free from the Apple App Store or Google Play Store. Over 500,000 users trust FirstVet to care for their animals. Rating: 4.9 - more than 1600 reviewsRating: 4.9 - more than 1300 reviewsRating: 4.9 - more than 1600 reviews Download app Horner Syndrome is caused by denervation of the sympathetic nervous system to the muscles around the eye. There are 2 main nerve pathways. First, the nerves go from the chest into the spinal cord, move into the skull and enter the brain, then move out of the brain and enter the eye. The other set of nerves exits the eye and moves along the neck below the spinal cord and enters the chest before entering the spinal cord. Injury or illness to any part of this nerve route can lead to Horner Syndrome.Clinical Symptoms of Horner SyndromeDogs and cats need to have three out of four of these symptoms in order to call it Horner Syndrome. Most often, the symptoms develop on one side of the face but rarely can occur on both sides simultaneously.Miosis (constricted/small pupil) on the affected side, the other pupil is often normal in sizePtosis (droopy eyelid)Enopththalmos (globe of the eye is receding back into the orbit)Third eyelid elevationCommon Causes of Horner Syndrome in Dogs and Cats Lesions in the brainstem, hypothalamus, or cavernous sinus leading to both sides (bilateral) of the face being affectedDiabetes mellitus can cause bilateral symptomsTrauma to head, neck, chest (car accident, fight wounds, etc.)Mass in the chest cavity, such as thymoma, lymphoma, or a hematomaMiddle ear disease (infection, polyp formation)Surgery around the earCancer of the thyroid gland, parathyroid gland, lungs, etc.Iatrogenic causes, such as blood draw, excessive restraint, nerve blocks around the face/neckNeosporosis infection (dogs)EmboliIdiopathic, meaning no underlying cause can be found. This makes up about 50% of cases.Testing for Horner Syndrome in PetsA complete physical exam, neurologic exam, and ophthalmic exam are needed to diagnose Horner Syndrome. If your dog or cat has 3 of the 4 symptoms present, then your vet will try to determine what is causing them to develop.Chest radiographs (x-rays) are often recommended to look for any masses in the chest or signs of trauma like broken ribs or vertebra.If the eardrum appears abnormal on exam, radiographs of the skull may be recommended to look for signs of inner or middle ear infections. If present, samples can be obtained for culture and sensitivity testing to identify the bacteria and help select the best antibiotic treatment option.If other neurologic symptoms are present, a spinal tap and fluid analysis may be recommended.An MRI of the skull and chest may be needed. These are more accurate at finding small lesions compared to radiographs.Finally, your vet may apply a special eye drop, phenylephrine, to the affected eye to monitor for signs of pupil enlargement. This can help localize where the lesion is since issues from the face to the chest can cause Horner Symptoms.Treatment Options for Horner Syndrome in PetsTreatment of Horner Syndrome depends on the underlying cause. If an inner or middle ear infection is present, appropriate flushing and antibiotic treatment can cure the infection and lead to resolution of the Horner Syndrome symptoms.If a tumor is found, surgery, radiation therapy, and chemotherapeutic options can be discussed with your vet.Most often (50% of cases in dogs and 40% of cases in cats), the cause is idiopathic, meaning we don’t know what the underlying cause is. Luckily, most of these patients start to have improvement in symptoms within 2 months, but it can take 6 months or longer. The symptoms often fully resolve, but may not in all cases. Due to the prolonged time to see improvement, screening for cancer and infections early is ideal as you don’t want to wait 2 to 6 months to wait and see how things go.Read more:A Vet’s Advice: Eye Exams and Eye Care for Your PetsSeizures in Dogs and CatsVestibular Disease in DogsHave more questions about Horner Syndrome in dogs and cats?Schedule a video consult to speak to one of our vets.